The present invention relates to a process for improving the color of prawns, shrimps and lobsters, and more particularly to a process which can change the color of prawns, shrimps and lobsters to pink, so as to avoid the common misapprehension that said shellfish are rotting simply because the original color of these shellfish is black brown, closely mimicking the color of a rotted prawn, shrimp or lobster.
There is no problem of confusion on the part of the buyer while said prawns, shrimps and lobsters, (hereinafter generally referred to as "prawns"), are sold fresh, due solely to the fact that the buyer may inspect the prawns and see that they are alive. In such a case, the buyer would not queston the color of the prawns, suspecting that they are not fresh. However, it is very difficult to transport a gross amount of prawns from far fishing grounds or prawn farms and sell them as fresh food owing to the limitations of transportation technology and price competition. Therefore, most prawns are transported in a frozen state.
To improve the color of prawns, a conventional method is used to process the prawns before the step for freezing them. That is, after the prawns are washed and classified, the prawns are heated in order to change astaxanthin, a king of natural carotenoid pigment, which exists within the prawn bodies, to a red substance-astacin. In this way, the prawn bodies will take on a pink color assuring buyers of the freshness of the product and therefore advancing the commercial value of the as-processed prawns.
Referring to FIG. 1, the thickness of the color change of the as-processed prawns is very different, depending on the degree of heat applied to them. FIG. 1 (a), FIG. 1 (b) and FIG. 1 (c) respectively show a prawn which turns red fully from the surface to the core, a prawn the red penetration of which comprises half of its whole thickness, and a prawn the red penetration of which comprises one third of its whole thickness. In these figures, H1, H2 and H3 respectively express depth of the red penetration. The red-penetrated thickness is preferably between 0.5-1 mm. If the depth of the red penetration is lower than 0.5 mm, the prawns may return to their original color, owing to the reduction in the reacted astaxanthin. As the depth of the red penetration in the prawn's body reaches a level higher than 1 mm, the body fluid of the prawns will be greatly reduced because of the associated heat treatment therefore causing the complete loss of any sense of freshness.
The temperature of washed and classified prawns is usually as low as 4 degrees centigrade. However, when they are treated by hot water of a temperature of nearly 100 degrees centigrade, the heat penetration rate will be very high and this heat remaining in the prawn body will enlarge the area of red penetraton radially, towards the core portion. Therefore, the regulation of the depth of red penetration is very difficult, depending almost entirely on the experience of a worker.
Towards this drawback, a suggestion is provided in the form of a lowering of the heat penetration rate by a reducing of the temperature of the hot water However, as the temperature of the hot water is lowered, the period for treating prawns will also have to be prolonged, allowing the heat of the water to have enough time to penetrate into the core portion of each prawn, so the inside of each prawn body will have a broad area that is partially penetrated by the red color.